1. Field of the Invention
The present invention relates to a semiconductor laser suitable for use as a light source apparatus in the field of optical frequency division multiplexing (FDM) communications, for example, and further relates to an optical communication system and a light source apparatus which use the above-mentioned semiconductor laser, respectively.
2. Related Background Art
In recent years, increased transmission capacity in the field of optical communications has become desirable, and the development of optical FDM communications, in which signals at a plurality of optical frequencies are multiplexed in a single optical fiber, has been advanced.
There are two kinds of optical FDM communication methods, which are classified by the type of light signal used in the receiving technique. One method is a coherent optical communication, in which a beat signal is produced between signal light and light from a local oscillator to obtain an intermediate-frequency output, and that output is detected. The other method is one, in which only light at a desired wavelength or optical frequency is selected by a tunable filter, and the thus-selected light is detected. The tunable filter used in the latter method may comprise one of a Max-Zehnder type, a fiber Fabry-Perot type, an acousto-optic (AO) modulator type, a semiconductor distributed feedback (DFB) filter type and the like. Especially, in the semiconductor DFB filter type, the transmission bandwidth can be narrowed down to less than 0.5 .ANG. and the optical amplification function (approx. 20 dB) exists, so the multiplicity of transmission wavelengths of signals can be increased and the minimum receiving sensitivity can be improved (i.e., the minimum receiving intensity can be reduced). An example of a semiconductor type filter is disclosed in T. Numai et al. "Semiconductor Tunable Wavelength Filter", OQE 88-65, 1988. Further, since this type of filter can be formed with the same material as a semiconductor photodetector, integration and downsizing are feasible. From the foregoing, the suitability of a semiconductor DFB type optical filter for optical FDM communications is clear.
On the other hand, in an optical communication system using a semiconductor filter as a light source in an optical transmitter, the semiconductor laser is required to have stable oscillation and polarization direction and to maintain a dynamic single mode. Therefore, as a semiconductor laser, DFB laser, distributed Bragg reflector (DBR) laser, or the like is used since they radiate only in the transverse electric (TE) mode. At present, the most popular modulation system for transmission signals in transmission systems is digital amplitude modulation, or amplitude shift keying (ASK) in which a drive current injected into a laser is directly modulated, or digital frequency modulation or frequency shift keying (FSK) in which a signal current having a minute amplitude is superposed on a bias current.
In the direct optical intensity or amplitude modulation system, however, the amplitude of a modulation current required for driving the laser needs to be large. In addition, the bias current point of the laser needs to be close to or less than its threshold. As a result, such a modulation system is unsuitable for high-frequency modulation. More particularly, fluctuation of its oscillation wavelength during the modulation of the semiconductor laser is relatively large. When the fluctuation surpasses the pass bandwidth of a tunable filter, the shape of demodulated signal is deformed, leading to an increase in an error rate of received signals and degradation of response characteristics during the high-frequency modulation state.
On the other hand, in the direct frequency modulation system, the channel width is narrow and hence the tracking control of a tunable filter needs to be accurately performed. Further, there is a tendency for crosstalk between wavelengths indicating codes "1" and "0" to occur depending on a change in surroundings, and an error rate of received signals increases. Moreover, since the FM modulation efficiency of a semiconductor laser needs to remain unchanged or flat over a wide range, the device is difficult to fabricate.
Optical communication apparatuses for solving the above problems are proposed in Japanese Patent Laind-Open Nos. 62(Showa)-42593, 62(Showa)-144426 and 2(Heisei)-159781. In those optical communication apparatuses, a DFB laser is caused to selectively output one of two polarization modes according to a transmission signal, and light emitted from the laser is transmitted through a polarizer and supplied to a light receiving device.
Details of those optical communication apparatuses will be described below.
FIG. 1 illustrates the schematic structure of the optical communication apparatus. In FIG. 1, reference numeral 101 designates a light source constructed of a DFB laser described below. A polarizer 102 is disposed in front of the light source 101. Light emitted from the light source 101 is transmitted through the polarizer 102, and is transmitted to a light receiving device 104 via an optical fiber 103. The light source 101 selectively outputs one of transverse electric (TE) and transverse magnetic (TM) modes based on a modulation signal from a signal source 105. Reference numeral 106 designates a bias current source. The polarizer 102 is disposed to transmit only one polarization mode of TE and TM modes.
FIGS. 2A and 2B illustrate a DFB laser used as the above-discussed optical communication apparatus. The DFB laser is fabricated as follows: A first order diffraction grating 112 is initially formed on an n-type InP substrate 110. On the grating 112, an n-type GaInAsP light waveguide layer 114, an undoped GaInAsP active layer 116, a p-type GaInAsP anti-meltback layer 118, a p-type InP clad layer 119 and a p.sup.+ -type GaInAsP ohmic contact layer 120 are formed in this order.
In an etching process, parts of the semiconductor layers 120, 119, 118, 116 and 114 and the first order diffraction grating 112 are removed and a mesa stripe portion 130 is formed. Then, a p-type InP layer 122, an n-type InP layer 123 and an undoped GaInAsP cap layer 124 are consecutively grown on the surroundings of the mesa stripe portion 130 to bury the surroundings. P-type electrodes 126a, 126b and 127 are deposited on the ohmic contact layer 120 and the cap layer 124, and an n-type electrode 128 is formed on the bottom surface of the substrate 110. Antireflection (AR) coatings 129a and 129b are provided on end facets of the cavity.
In the thus-fabricated DFB laser, when a constant bias current is injected through the electrodes 126a and 126b and a predetermined current is injected through the electrode 127, the DFB laser oscillates in TM mode. When the polarizer 134 disposed on the emission side of the DFB laser is set such that the TM mode oscillation light cannot be transmitted through the polarizer 134, output light is blocked by the polarizer 134 and no light can be taken out (off-state). When the current flowing through the electrode 127 is adjusted and an amount of equivalent phase shift is changed, TE mode light begins to be output. At this time, the output light can be taken out from the polarizer 134. In the optical communication apparatus, since the area of the electrode 127 is small, the DFB laser can be operated at a high speed by a small modulation current of about 10 mA.
However, the conventional arts do not inform with respect to minimum values of threshold gain of the active layer between TE and TM modes in the above DFB layer. In the case that a difference in minimum values of threshold gain is large, this reduces the decrease of the modulations drive current in the optical communication method in which the polarization mode is switched. Therefore, dynamic wavelength fluctuation in a high frequency range is not so highly improved, even compared with the ASK modulation.